The present invention relates to a method of forming a thin polycrystalline silicon film by a plasma CVD method as well as a thin film forming apparatus which can be used for forming a thin polycrystalline silicon film.
Thin silicon films have been used as a material for TFT (thin film transistor) switches arranged in pixels of liquid crystal displays, and have also been used for producing those such as various kinds of integrated circuits and solar batteries.
The thin silicon films are formed by a plasma CVD method using silane-containing reaction gases in many cases. Most of such thin films are thin amorphous silicon films.
A thin amorphous silicon film can be formed on a film deposition substrate which is kept at a relatively low temperature, and the thin film having a large area can be easily formed in a plasma of a material gas produced by radio-frequency discharging (frequency: 13.56 MHz) using electrodes of a parallel plate type. Owing to the above, the thin amorphous silicon films have been widely used in switching devices for pixels of liquid crystal displays, solar batteries and the like.
However, an electric power generation efficiency of a solar battery using a silicon film as well as characteristics such as a response speed of a semiconductor device using a silicon film cannot further be improved as long as an amorphous silicon film is used. Accordingly, it has been studied to utilize a thin crystalline silicon film such as a thin polycrystalline silicon film.
A thin crystalline silicon film such as a thin polycrystalline silicon film can be formed by various methods, in which a film deposition substrate is kept at a temperature from 600xc2x0 C. to 700xc2x0 C., such as CVD methods (e.g., a low pressure plasma CVD method and a thermal CVD method), and PVD methods such as a vacuum vapor deposition method and a sputtering vapor deposition method. Also, the film may be formed, first by forming a thin amorphous silicon film at a relatively low temperature by a method selected from various kinds of CVD and PVD methods, followed then by a post-treatment of effecting a thermal treatment at about 800xc2x0 C. or more, or of effecting a thermal treatment at about 600xc2x0 for a long time.
Such a method is also known that laser annealing is effected on an amorphous silicon film for crystallizing it.
According to a method in which a substrate is subjected to a high temperature, however, it is necessary to employ an expensive substrate which can withstand a high temperature. For example, it is difficult to form a thin crystalline silicon film on an inexpensive glass substrate having a low melting point (and a heat resistance temperature of 500xc2x0 C. or less). Accordingly, a thin crystalline silicon film such as a thin polycrystalline silicon film requires a high manufacturing cost.
According to a laser annealing method, a thin crystalline silicon film can be produced at a low temperature. However, a laser emitting step is required, and laser light must be emitted with an extremely high energy density. For those and other reasons, a manufacturing cost of a thin crystalline silicon film becomes high.
Accordingly, an object of the invention is to provide a method of manufacturing a thin polycrystalline silicon film at a relatively low temperature with a low cost and a high productivity.
It is also an object of the invention to provide a thin film forming apparatus, which can manufacture a thin polycrystalline silicon film at a relatively low temperature with a low cost and a high productivity, and can furthermore be widely utilized for forming a desired thin film.
For achieving the above objects, the inventors made studies, and obtained the following knowledge:
A gas mixture, supplied into a film deposition chamber, of a material gas having silicon atoms [e.g., a silicon tetrafluoride (SiF4) gas or a silicon tetrachloride (SiCl4) gas] and a hydrogen gas or a silane-containing reaction gas [e.g., a mono-silane (SiH4) gas, a disilane (Si2H6) gas or a trisilane (Si3H8) gas], supplied into a film deposition chamber, is decomposed by plasma formation so that a large number of decomposition products (various radicals or ions) are formed. Among the decomposition products, SiH3*, SiH2*, SiH* and the like are the radicals that contribute to formation of a thin silicon film. The structure of the film, decided in the process of growth of the thin silicon film, depends on a surface reaction on a substrate. It is considered that film deposition occurs as a result of reaction between uncombined hands of silicon atoms present on the substrate surface and the radicals. For crystallizing the thin silicon film, it is necessary to suppress as much as possible such a situation that silicon atoms having uncombined hands or hydrogen atoms combined with silicon atoms are taken into the film. For that, it is considered important to increase the covering rate of hydrogen atoms over the substrate surface and over the film surface formed thereon. Detailed mechanism is unknown on how hydrogen atoms covering the substrate surface and the film surface formed thereon suppress those such as hydrogen atoms combined with silicon atoms having uncombined hands taken into the film. However, it is considered that the uncombined hands of silicon atoms are sufficiently combined with hydrogen causing vaporization of them. In summary, increase in the covering rate of hydrogen atoms over the substrate surface reduces silicon atoms having uncombined hands as well as hydrogen atoms combined with silicon atoms taken into the substrate. For increasing the covering rate of hydrogen atoms over the substrate surface, it is essential that hydrogen atom radicals are always emitted from a plasma to the substrate. For this, it is important to increase the density of hydrogen atom radicals in the plasma. According to studies by the inventors, a thin polycrystalline silicon film of good quality can be formed by increasing the density of hydrogen atom radicals so that the ratio of an emission intensity of hydrogen atom radicals (Hxcex2) to an emission intensity of SiH* radicals in the plasma, [i.e., (emission intensity of hydrogen atom radicals (Hxcex2))/(emission intensity of SiH* radicals)] may be one or more.
The invention is based on the above knowledge, and provides a thin polycrystalline silicon film forming method and a thin film forming apparatus described below.
(1) Thin Polycrystalline Silicon Film Forming Method
A method of forming a thin polycrystalline silicon film, in which a plasma is formed from a gas mixture of a material gas having silicon atoms and a hydrogen gas, or from a silane-containing reaction gas, the state of the-plasma is controlled to provide an emission intensity of hydrogen atom radicals (Hxcex2) in the plasma exhibiting a ratio of one or more to an emission intensity of SiH* radicals, and a thin polycrystalline silicon film is formed on a substrate in the plasma.
(2) Thin Film Forming Apparatus
A thin film forming apparatus of a plasma CVD type including a film deposition chamber for accommodating a deposition target substrate; a discharging electrode for plasma formation, in the film deposition chamber, connected to a discharging power source; a gas supply device for supplying a gas into the deposition chamber for film deposition; and an exhaust device for exhausting a gas from the deposition chamber, wherein the apparatus further includes an emission-spectrometer and a probe measuring device for measuring the plasma state, and a control portion for controlling at least one of a power supply from the discharging power source (typically, a magnitude of the supplied power), a gas supply from the gas supply device (typically, a supplied gas flow rate) and exhausting by the exhaust device for maintaining a predetermined plasma state based on information detected from the emission-spectrometer and the probe measuring device.